1. Field of the Invention
The present invention relates to a liquid crystal display device, and, more particularly, to a electrically controlled birefringence (ECB) type liquid crystal display device capable of displaying full-color images.
2. Description of the Related Art
Color liquid crystal display (LCD) devices which display color images generally use color filters.
A color LCD device using this color filter suffers dark display due to the light absorbed by the color filter. When a color filter is used in a reflection type LCD device which is not equipped with a back light, particularly, the light is absorbed twice, at the incident time and the outgoing time, so that the display becomes very dark. It was therefore difficult to provide a reflection type color LCD device using a color filter.
Under this situation, recently, ECB type color LCD apparatuses which use no color filters and which, if they are of a reflection type, can present sufficient bright display.
An ECB type color LCD device comprises a liquid crystal (LC) cell, which is comprised of a pair of substrates which have electrodes formed on their opposing surfaces and a liquid crystal like a TN liquid crystal sealed between the two substrates, a pair of polarization plates sandwiching the LC cell, and a reflector provided outside one of the polarization plates. At least one retardation plate may be provided between the LC cell and the polarization plates.
In the ECB type color LCD apparatus, linearly polarized light which has passed one of the polarization plates becomes elliptically polarized light whose polarization state differs wavelength by wavelength, while passing the LC cell (and the retardation plate). As this elliptically polarized light passes the other polarization plate, the wavelength components of the light are selectively passed, thus presenting a display color. The birefringence of the liquid crystal can be controlled by controlling the voltage to be applied between the electrodes of the LC cell. This changes the polarization state of the light passing the LC cell to acquire the light with the desired color. It is therefore possible to display a plurality of colors with a single pixel.
The conventional ECB type color LCD apparatus however has the following problems.
1. The ECB type color LCD apparatus which uses conventional polarization plates suffers "white" becoming yellowish by the birefringence effect of the liquid crystal. To make this yellowish "white" to pure "white," a bluish polarization plate whose blue color is a complementary color to yellow may be used. Even with such a polarization plate, "white" still becomes yellowish. Moreover, the display of "black" becomes bluish, not dark.
2. Since the conventional ECB type color LCD apparatus cannot display pure and bright "white" and pure and dark "black," the display contrast is low. Further, this conventional ECB type color LCD apparatus cannot clearly display white and black, which are the basic display colors, and three primary colors of red (R), green (G) and blue (B) with high purities.
3. When a voltage is applied between the electrodes, the areas around the electrodes (non-display areas) show electro-optical responses different from those of the areas which display colors in accordance with the applied voltage. The display colors of the non-display areas therefore differ from the display colors of the electrode areas (display areas), thus lowing the purities of the display colors and the display contrast. To overcome this problem, a black mask is formed on the common electrodes (in the case of an active matrix LCD device) to shield the display of the non-display areas. In this case, however, possible misalignment of the upper and lower substrates may cause the black mask to cover the display areas to thereby reduce the effective area of the display areas, thus reducing the brightness of the displayed image.
4. As the display color and gradation (brightness) simultaneously change with respect to the applied voltage in the conventional ECB type color LCD apparatus, it is difficult to ensure gradation display and multi-color display. As a solution, an ECB type color LCD device and a monochromatic LCD device may be placed one on the other so that the light whose gradation is obtained by the monochromatic LCD device is colored by the conventional ECB type color LCD device, thus presenting gradation display for each color. Such a double LCD-device structure is complex and requires approximately two times the cost of a single LCD-device structure.